Regularly receiving suspicious certificate errors online

For the past few weeks, I have frequently been receiving error messages from websites stating that the certificate is invalid. This tends to happen for a while and then resolve itself. Other devices on the same network connection are also experiencing odd behaviour, including a very sporadic internet connection that is either very slow or turns on and off. We had an engineer visit this week to diagnose whether there was a faulty connection, but they left, happy that the connection is working properly.

I understand that receiving ‘bad’ certificates can sometimes be a sign of the system clock or internet settings being poorly configured. I had assumed this might be the case, until I interrogated one of these ‘invalid certificate’ messages more closely (from Firefox on Mac):

Web sites prove their identity via certificates. Firefox does not trust this site because it uses a certificate that is not valid for eur03.safelinks.protection.outlook.com. The certificate is only valid for the following names: cloudflare-dns.com, *.cloudflare-dns.com, one.one.one.one, 1.1.1.1, 1.0.0.1, <…>

The website I was trying to visit in this case has nothing to do with cloudflare, and the links above lead to a website that appears to be selling a VPN-type service.

Should I be concerned that my internet connection has been tapped, and what would be the appropriate action to take to shake this off?